1. Field of the Invention
This invention relates to a zoom lens suitable for a television camera and a video camera, and, more particularly, to a zoom lens equipped with a temperature compensating feature in which an extender lens group is removably incorporated to increase an effective focal length of the zoom lens.
2. Description of Related Art
There have been known a variety of zoom lenses for television cameras, video cameras and so forth which are designed and adapted to make a shift of focus as small as possible under various environmental conditions, in particular under a wide range of temperature conditions. In order to prevent aggravation of optical performance of such a zoom lens due to a shift of focus caused by a change in circumferential temperature, it has been known to make a selection of expansion coefficients for lens holding members and the like such that they are most suitable for causing deformation according to a change in circumferential temperature so as to shift lens elements in an axial direction opposite to a shift of focus due to the change in circumferential temperature and thereby to cancel the shift of focus of the zoom lens as one whole.
It is usual to change or increase the effective focal length of the zoom lens in order to achieve increased magnification, in particular, to produce an enlarged optical image of a subject at a long distance. As a simple way of increasing the effective focal length, it has been known to use an extender or tele converter lens group (which is hereafter referred to as an extender lens group) that increases the effective focal length of the zoom lens when incorporated in the zoom lens. The zoom lens is ordinarily adjusted to show the best optical performance under normal temperature conditions. In the case where the zoom lens is adapted to use an extender lens group in order increase its effective focal length, the zoom lens is adjusted such that the focal point is immovable under normal temperature conditions while the extender lens group is incorporated. However, when the circumferential temperature condition changes from normal temperatures, a component lens element changes its curvature of radius and index of refraction according to a coefficient of linear expansion and a change in the temperature coefficient of refractive index of the component lens element, respectively. Further, an axial distance between each adjacent component lens elements changes according to their coefficients of linear expansion. In consequence, the zoom lens causes a shift of focus between when the extender lens group is incorporated and when it is removed under circumferential temperature conditions other than normal temperatures.
Typically, the zoom lens has a front group of lens elements, namely a focusing lens group, which is movable along the optical axis to achieve focusing from infinity to moderately close distances. This focusing lens group functions to correct a shift of focus due to a change in temperature. However, when the zoom lens is focused on a subject while the extender lens group remains removed, it has to be readjusted to focus on the subject after incorporating the extender lens group. In the event where the zoom lens is focused on the side of the shortest zoom setting, i.e. on the wide angle side, since a shift of focus that is corrected by the focusing lens group is a little because of optical structure of the zoom lens, it is hard for the focusing lens group to correct an overall shift of focus caused by a change in temperature. Although the rearmost lens group is possibly used to focus the zoom lens, it is hard to correct a shift of focus due to incorporation of the extender lens group. In view of the above, in order for the zoom lens to show desired optical performance, it is essential that the zoom lens is designed such that the zoom lens with the extender lens group incorporated therein does not cause a shift of focus due to a change in circumferential temperature when it is focused on the wide angle side.
For the purpose of providing a brief background that will enhance an understanding of the effect of a zoom lens of the present invention, reference is made to FIG. 7.
FIG. 7 shows a change of modulation transfer function (MTF) with respect to a shift of focus caused due to a change in temperature. As shown by solid lines (X1, X2), under normal temperature conditions, the zoom lens without the extender lens group has a focal point which coincides with a focal point of the zoom lens with the extender lens group. As shown by a chained line, while the zoom lens incorporates the extender lens group under temperature conditions other than normal temperature conditions, it causes a shift of focus from the focal point of the zoom lens without the extender lens group. In consequence, the zoom lens with the extender lens group experiences aggravation of modulation transfer function (MTF) as compared with the zoom lens without the extender lens group.